1. Field of the Invention
This invention relates to an evaporator control method and an apparatus therefor to control condition of raw solution which is heated and concentrated in the evaporator in a salt manufacturing plant.
2. Description of the Prior Art
In a salt manufacturing plant, raw solution accommodated in an evaporator is heated to evaporate water so that salt is precipitated and deposited from a concentrated raw solution.
In the salt manufacturing process, the residual solution becomes rich since it becomes bitter and potassium chloride is precipitated when the final concentration becomes high in the bitter solution, and this reduces the precipitated and deposited quality of the salt. Therefore, the bitter solution should be discharged and new raw solution should be fed to the evaporator. As a result, the salt is continuously produced by adjusting the temperature of the solution in the evaporator so that potassium chloride and the like are not precipitated.
In a crystallizing process of concentrating the raw solution, the solution before the salt is precipitated is called "brine" and the solution is called "mother solution" once it reaches the condition that salt is precipitated. When the solution reaches the condition in which potassium chloride, magnesium chloride, and calcium chloride are precipitated it is called "bitter solution". Precipitation from the mother solution is optimal to obtain high quality salt.
In order to control the concentration of the raw solution which is boiled in the crystallizing process, it is required to determine the condition of the raw solution in the evaporator. However, it takes a long time to take samples from the raw solution and to determine the concentration of the raw solution through manual operation, that is, a lot of time is required from the sampling to the determination of the concentration, resulting in large fluctuations of the final boiling concentration. Therefore, the measurement of the concentration in real time should be automatically carried out to obtain the concentration to prevent the fluctuation thereof.
As a method of automatically measuring the concentration of the raw solution, it may be possible to introduce the raw solution into a pipe to measure the concentration with an ultrasonic concentration meter. The ultrasonic concentration meter measures the concentration based on the decrease of the frequency of an oscillated ultrasonic wave, as is disclosed in Japanese patent application laid-open Showa 54-128395 and 55-57146.
As described above, it is possible to measure the concentration of the raw solution in real time utilizing an oscillated ultrasonic wave. However, the concentration can not be controlled only through the concentration of the overall raw solution because a calibration curve which shows the relationship between the concentration of the overall raw solution and the sound speed varies according to the temperature. Therefore, the temperature of the raw solution as well as the concentration thereof should be taken into consideration.
In order to improve the efficiency of the salt manufacturing process, the concentration of the raw solution must be controlled in such a manner as to directly determine the condition of the raw solution i.e., such as "brine", "mother solution" or "bitter solution".